Another witness Connor Neil said people were being evacuated from their homes and there was a “big orange light" which could be seen from across the city centre.

The 22-year-old chef from Glasgow said: “We’ve got just to the top of Renfrew Street and police closed off the road.

“You can just see a big orange light coming from the school and people are being evacuated from their homes.”

Fire took hold of the Glasgow School of Art building in May 2014 in the run-up to the students’ degree show, and graduation ceremonies had taken place at the institution earlier on Friday.

A restoration project, which was set to cost between £20 million and £35 million, had been returning the famous art school to its former glory, with Brad Pitt and Peter Capaldi among those lending their support.

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RUINS: Forensic archaeologists sifts through the ashes of the fire damaged library in 2014

The fire at the building, which was designed by artist and architect Charles Rennie Mackintosh and built between 1897 and 1909, has been called "devastating" by the city council leader Susan Aitken.

Paul Sweeney, shadow Scotland minister, said the Mackintosh Building was the "most architecturally important building" in the city.

He added: "Oh dear, the 1909 library extension, that was the origin of the 2014 blaze is now fully alight too. It looks like the entire interior space is now fully alight.

"The best we can probably hope for is structural facade retention and a complete rebuild of the interior. Devastating."

The worst fires ever to hit Britain

Dramatic pictures which rocked a nation.

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Grenfell Tower is thought to be one of the worst fires ever seen, 2017

Scottish First Minister Nicola Sturgeon said she was heartbroken by the fire and paid tribute to the emergency services.

She tweeted: "This is clearly an extremely serious situation. My first thoughts tonight are for the safety of people - but my heart also breaks for Glasgow's beloved @GSofA.

"Once again, we are indebted to the bravery of our firefighters and other emergency services."